1. Field of the Invention: This invention relates to the field of ceramics and particularly to alumina-zirconia composite (Al.sub.2 O.sub.3 -ZrO.sub.2) ceramics.
2. Description of the Prior Art: High toughness ZrO.sub.2 ceramics have been fabricated by taking advantage of the volume increase which accompanies the change from tetragonal to monoclinic transformation. This "transformation toughening" (F. Lange, J. Mater. Sci., 17 225-263 (1982)) increases the energy required for crack propagation. U.S. Pat. No. 4,316,964 to Lange increased the fracture toughness of Al.sub.2 O.sub.3 -ZrO.sub.2 ceramics by dissolving a rare earth oxide such as Y.sub.2 O.sub.3, CeO.sub.2, La.sub.2 O.sub.3 and/or Er.sub.2 O.sub.3 in the ZrO.sub.2 so as to stabilize metastable tetragonal ZrO.sub.2 at low temperatures.
Hafnium Oxide (HfO.sub.2) undergoes a martensitic transformation similar to that of ZrO.sub.2 except that the monoclinic to tetragonal transformation temperature increases and the volume expansion for the tetragonal to monoclinic transformation decreases. Due to the lack of commercially available HfO.sub.2 powders, ZrO.sub.2 -based ceramics are used in applications such as insulators, values, die-liners, cutting tools and wear parts. Since ZrO.sub.2 and HfO.sub.2 form a solid solution, mixtures of zirconia and hafnia can be used to control the transformation temperature.
Two of the most commonly used additives for stabilizing tetragonal ZrO.sub.2 are Y.sub.2 O.sub.3 and CeO.sub.2. ZrO.sub.2 -based ceramics stabilized with Y.sub.2 O.sub.3 generally have higher strength but lower toughness than ZrO.sub.2 -based ceramics stabilized with CeO.sub.2. Such ceramics are generally classified as tetragonal zirconia polycrystals "TZP" due to their fine-grained microstructure which is nearly all tetragonal at room temperature.
The strength of yttria TZP (Y-TZP) and ceria TZP (CeTZP) ceramics increases with alumina additions and the fracture toughness correspondingly decreases (K. Tsukuma and T. Takahata, "Mechanical Property and Microstructure of TZP and TZP/Al.sub.2 O.sub.3 Composites," Advanced Structural Ceramics, Vol. 78, ed. by P. F. Becher, M. V. Swain and S. Somiya (Materials Research Society, Pittsburgh, PA, 123-135, 1987)).
The maximum in fracture toughness for both Y-TZP/Al.sub.2 O.sub.3 and Ce-TZP/Al.sub.2 O.sub.3 composites occurred at approximately 30 volume percent (vol. %) Al.sub.2 O.sub.3. In the case of Y-TZP/Al.sub.2 O.sub.3 ceramics, Tsukuma add Takahata found that strength increased from 1.5 gigapascals(GPa) to 2.4 GPa while fracture toughness simultaneously decreased from 11 MPa.ml/.sup.1/2 to 6 MPa.ml/.sup.1/2 with increasing Al.sub.2 O.sub.3 up to 30 vol. %. The same investigators found similar results with Ce-TZP/Al.sub.2 O.sub.3 ceramics in that strength increased from 200 MPa to 900 MPa while fracture toughness simultaneously decreased from 32 MPa.ml.sup.1/2 to 14 MPa.m.sup.1/2 with increasing Al.sub.2 O.sub.3 up to 30 vol. %. While absolute numbers are not crucial since testing methods may influence the data, the general trends are important and show the strength fracture toughness trade-off which exists in ZrO.sub.2 -based ceramics.
Al.sub.2 O.sub.3 has low strength and toughness as compared to ZrO.sub.2 ceramics. Al.sub.2 O.sub.3 /ZrO.sub.2 ceramics are attractive, as compared to ZrO.sub.2 ceramics, for high temperature applications since alumina has better retention of mechanical properties (i.e., strength and toughness) as a function of temperature than zirconia. The strength and toughness of TZP materials decrease rapidly with temperature, since the stability of the tetragonal polytype is increased, thereby making transformation toughening more difficult. Additionally, alumina additions to zirconia are beneficial in applications where creep resistance is required and certain applications where the erosion or wear resistance of the ceramic is not controlled by the fracture toughness. The higher hardness of Al.sub. O.sub.03/ ZrO.sub.2 composites in these applications generally relates to improved wear or erosion resistance. Alumina has higher thermal conductivity and lower thermal expansion than zirconia which is advantageous in limiting thermal shock. It would therefore be an improvement in the art if fracture toughness could be maintained while adding alumina to TZP ceramics and if high toughness Al.sub.2 O.sub.3/ ZrO.sub.2 ceramics could be made with increasing amounts of Al.sub.2 O.sub.3. Furthermore, it would be an improvement in the art if Al.sub.2 O.sub.3/ ZrO.sub.2 ceramics could be made without the traditional strength-fracture toughness trade-off. In the following summary of the invention it is given that HfO.sub.2 (or HfO.sub.2. ZrO.sub.2 solid solutions) can be substituted for ZrO.sub.2.